EI 11C201
CEMP-E
1 March 1997
to 100,000 gpd/mile) of pipe may be used depending on the size and age of the sewers,
materials of construction, and the soil and ground-water conditions. Installation personnel will
usually have some knowledge of these matters and should be aware of major problems. Where
I/I is known to be excessive, it should be determined prior to design if corrective measures are
planned for the existing system, or if U.S. Environmental Protection Agency (EPA) evaluation and
rehabilitation programs will be implemented.
d. Industrial waste flows. Industrial waste quantities from ordnance plants, technical
laboratories, laundries, vehicle maintenance shops, airplane wash racks, plating shops, and such
industries cannot be computed totally on a population or fixture unit basis. Flows from such
plants depend upon the type and extent of the activities. Industrial waste sewers and sanitary
sewers will be designed for the peak industrial flow as determined for the particular industrial
process or activity involved.
e. Fixture unit flow. The size of building connections, including those from theaters,
cafeterias, clubs, quarters, and other such buildings, will in all cases be large enough to
discharge the flow computed on a fixture unit basis as set forth in the manual on plumbing, TM
5-810-5/AFM 88-8, chapter 4. This requirement applies to building connections only, and not to
the lateral or other sewers to which they connect.
3-2. GRAVITY SEWER DESIGN. Sewers will be designed to discharge the wastewater flows as
required by paragraph 3-1. Generally, it is not desirable to design sewers for full flow, even at
peak rates. Flows above 90 to 95 percent of full depth are considered unstable, and may result
in a sudden loss of carrying capacity with surcharging at manholes. In addition, large trunk and
interceptor sewers laid on flat slopes are less subject to wide fluctuations in flow, and if designed
to flow full may lack sufficient air space above the liquid to assure proper ventilation. Adequate
sewer ventilation is a desirable method of preventing the accumulation of explosive, corrosive or
odorous gases, and of reducing the generation of hydrogen sulfide. Therefore, trunk and
interceptor sewers will be designed to flow at depths not exceeding 90 percent of full depth;
laterals and main sewers, 80 percent; and building connections, 70 percent. However,
regardless of flow and depth the minimum sizes to be used are 150 millimeter (6-inch) for
building connections and 200 millimeter (8-inch) for all other sewers. Building connections that
do not carry sanitary waste and will transport liquids with little or no solids, such as condensate
lines, can be smaller than 150 millimeters (6 inches), but no smaller than 100 millimeters (4
inches) is recommended for most situations. Industrial applications will use the same design
criteria as sanitary sewers except pipe material that is resistant to the waste will be specified.
The following formula, charts, procedures and criteria will be used for design.
a. Design formula and charts. The Manning formula will be used for design of gravity flow
sewers as follows:
C
)))))) R2/3S1/2
V
=
n
where:
C
=
1 for SI units (1.486 for IP units)
V
=
velocity in meters per second (feet per second)
n
=
coefficient of pipe roughness
R
=
hydraulic radius in meters (feet), and
S
=
slope of energy line in meters per meter (feet per foot)
3-4
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